Ceramic foam filters (CFF) formed from reticulated polyurethane foam exist. These CFFs are formed by impregnating the reticulated polyurethane foam with a ceramic slip, removing the excess slip by squeezing the foam, and then drying and firing the body, forming a CFF.
However, these CFFs create a number of problems in the production of the CFFs and in the application of the CFFs. In the production of CFFs, because the CFFs are formed via reticulated polyurethane foam, a precise pore size and shape of the reticulated polyurethane foam is difficult to control. One difficulty is controlling the pore size and shape of one batch of reticulated polyurethane foam to another pore size and shape of another batch of reticulated polyurethane foam. Additionally, the reticulated polyurethane foam offers only one pore size and shape across a cross-section of the CFFs. For example, the reticulated polyurethane foam offers a single pore size and shape relative to a depth of the CFFs. Stated otherwise, the reticulated polyurethane foam offers a single pore size and shape along a direction of flow of a liquid flowing through the CFFs.
In another example of the difficulty in producing CFFs, because the underlying reticulated polyurethane foam is ablated or burnt away, the CFFs are weak or brittle and the polyurethane foam is detrimentally lost to the environment. For example, the ablated reticulated polyurethane foam is exhausted out from the ceramic encasing the underlying reticulated polyurethane foam which produces undesirable whiskers of the ceramic. The undesirable whiskers break off from the CFFs, producing debris. In another example, the ablated reticulated polyurethane foam produces a hollow core of the ceramic. For example, the ablated reticulated polyurethane foam leaves a void in the ceramic. The hollow core of the ceramic weakens the resulting CFFs.
In another example of the difficulty in producing CFFs, because the polyurethane foam is impregnated with a ceramic slip, the polyurethane foam is limited to a maximum height or thickness (e.g., a maximum height of about 50 millimeters). For example, the polyurethane foam is limited to a maximum height because the ceramic slip must be able to be dispersed throughout the entire body of polyurethane foam, and subsequently able to be removed from the entire body of polyurethane foam, by squeezing the polyurethane foam.
CFFs formed of multiple layers of polyurethane foam are possible, but have distinct interfaces between each layer. For example, a first layer having a first pore size and shape may be fixed to, and interface with, a second layer having a second pore size and shape. However, the interface between the two different layers produces an irregularity that reduces a performance of the multiple layered CFFs. In addition to the interface irregularity, multiple layered CFFs are difficult to produce and are expensive.
In the application of CFFs, because the CFFs are weak or brittle the CFFs are difficult to replace. For example, in molten aluminium casting using CFFs, a new or unused CFF is installed in a filter box, one-drop or dose of molten aluminium is filtered through the CFF, and subsequent to the one-drop of molten aluminium the used or spent CFF is removed. Because the CFFs are weak or brittle, it is difficult for a user to remove them from the filter box after the one-drop of molten aluminium. For example, a user may attempt to grasp the spent CFF with a tool (e.g., tongs, pliers, skewers, etc.) to remove the CFF, but because the CFF is weak the CFF may break apart into multiple fragments due to a force applied to the CFF by the tool, making removal of the CFF difficult.
In another example of difficulty in applying CFFs, because the CFFs have a single pore size the process of casting molten aluminium using CFFs is strictly controlled to optimize filtration efficiency. For example, because of fine pore filtration (e.g., up to 80 grade) and high metal flow rates (e.g., up to 19 millimeters/second) demanded by the industry of molten aluminium casting, operating parameters of the filter systems (e.g., filter system preheating, filter system bowl design, filter system bowl sizing, etc.) are strictly controlled to ensure optimal filtration efficiency is achieved. In addition, the correct CFF size and grade is strictly controlled to ensure optimal filtration efficiency is achieved.
In addition to CFFs, other ceramic filters exist. However, the other ceramic filters have disadvantages. For example, deep or packed bed filters exist that offer medium to high efficiencies, but are very large and expensive to maintain compared to CFFs. In another example, bonded particle tube filters exist that offer very high filtration efficiencies, but are very large, have very expensive filtration media, and require long pre-heat times. In comparison, CFFs systems provide medium to high filtration efficiency, are compact, use lower cost filtration media (i.e., low cost CFFs), and have rapid pre-heat times. Subsequently, filtration of aluminium with CFFs in a casthouse has become the standard throughout the industry of molten aluminium casting to meet demand for an ever-increasing quality of semi-finished wrought aluminium products.
Thus, there remains a need to develop new ceramic filters, not formed from reticulated polyurethane foam.